The invention relates generally to diagnostic imaging and, more particularly, to a method and apparatus for fast dual-kVp switching in existing CT imaging systems.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a cone-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are quantized and transmitted to a data processing system for analysis, which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam from a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator adjacent the collimator for converting x-rays to light energy, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.
Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are quantized then transmitted to the data processing system for image reconstruction.
A CT imaging system may include an energy discriminating (ED), multi energy (ME), and/or dual energy (DE) CT imaging system that may be referred to as an EDCT, MECT, and/or DE-CT imaging system. The EDCT, MECT, and/or DE-CT imaging system is configured to be responsive to different x-ray spectra. For example, a conventional third-generation CT system may acquire x-ray projection data sequentially at different peak kilovoltage (kVp) levels of the x-ray source, which changes the peak and spectrum of energy of the incident photons comprising the emitted x-ray beams. Energy sensitive detectors may be used such that each x-ray photon reaching the detector is recorded with its photon energy.
Techniques to obtain the measurements include scanning with two distinctive energy spectra, and detecting photon energy according to energy deposition in the detector. EDCT/MECT/DE-CT provides energy discrimination and material characterization. For example, in the absence of object scatter, the system derives the behavior at a different energy based on the signal from two regions of photon energy in the spectrum: the low-energy and the high-energy portions of the incident x-ray spectrum. The detected signals from two energy regions provide sufficient information to resolve the energy dependence of the material being imaged. Furthermore, detected signals from the two energy regions provide sufficient information to determine the relative composition of an object composed of two hypothetical materials.
A principle objective of dual-energy scanning is to obtain diagnostic CT images that enhance contrast separation within the image by utilizing two scans at different energy spectra. For industrial inspection systems, the information allows characterization of the material-specific properties of scanned objects. A number of techniques have been proposed to achieve dual energy scanning, using a non-energy-discriminating detector such as a scintillator. Such techniques may include acquiring two scans either back-to-back sequentially in time where the scans require two rotations around the subject, or interleaved as a function of the rotation angle requiring one rotation around the subject, in which the tube operates at, for instance, 80 kVp and 140 kVp potentials. High frequency generators have made it possible to switch the potential of the x-ray source on alternating views. As a result, dual-energy projection data (high- and low-energy projection data) may be obtained in a temporally interleaved fashion, rather than two separate scans made several seconds apart, as required with previous CT technology. Taking separate scans at several seconds apart from one another results in mis-registration between datasets caused by patient motion (both external patient motion and internal organ motion) and, in the case of helical scanning such as used for inline object inspection, different cone angles. And, in general, a conventional two-pass dual kVp technique cannot be applied reliably where small details need to be resolved for body features or object features that are in motion, such as in a baggage scanner.
While switching the x-ray source potential via the high frequency generators solves many of the problems related to conventional two-pass dual-energy scanning, such a configuration does not always provide the switching speed needed for certain imaging applications. For example, cardiac imaging and certain low-current imaging applications such as security and industrial inspection cannot be effectively performed by simply switching the x-ray source potential with the high frequency generator. Often, there is a delay in the response time of the switched operating potential between the high frequency generator and the x-ray source, due in part to the capacitance of the cable and the x-ray tube connecting these devices, and the low x-ray tube current levels.
Using the dual-kVp switching technique described above, a high-voltage generator is coupled directly to an x-ray source, such as an x-ray tube, via a high-voltage cable. While the high-voltage generator may be a dual-stage generator capable of switching between two distinct voltage levels to perform dual-kVp imaging, this switching is typically affected by capacitive and other effects. That is, the rise time in switching the generator from a first (low) voltage, or first kVp, level to a second (high) kVp level is limited by the power of the high-voltage generator and may be too slow for effective dual-kVp imaging in many medical and security applications. Likewise, the fall time between switching the high kVp to a low kVp level is generally very slow, which effectively reduces the energy separation of the applied spectra, resulting in reduced material specification sensitivity and, therefore, the effectiveness of the dual-kVp imaging. As such, these insufficient switching speeds often lead to projection data pair mis-registration and streak artifacts in reconstructed images.
Therefore, it would be desirable to design an apparatus and method to provide suitable dual-kVp switching.